In a fuel injection system provided with a pressure control chamber formed on an inside end of a needle valve and with an intermediate chamber of a booster piston for increasing the injection pressure, discharging high pressure fuel in a common rail supplied into the pressure control chamber to the inside of a fuel discharge passage so as to open the needle valve and inject fuel, and discharging high pressure fuel in the common rail supplied into the intermediate chamber into the fuel discharge passage so as to operate the booster piston and increase the fuel injection pressure, known in the art is a fuel injection system designed to connect the pressure control chamber and intermediate chamber through a three-position switching type three-way valve to the fuel discharge passage and to use the switching action of this three-way valve to connect both the pressure control chamber and intermediate chamber to the fuel discharge passage when increasing the injection pressure at the time of fuel injection and connect only the pressure control chamber to the fuel discharge passage when not increasing the injection pressure at the time of fuel injection, that is, when stopping the operation of the booster piston (see Japanese Patent Publication (A) No. 2003-106235).
However, in the above-mentioned three-position switching type three-way valve, the excitation current supplied to the electromagnetic coil for driving the valve element is changed so as to make the valve element move to either one end position, an intermediate position, or another end position. In this case, electromagnetic force may theoretically be used to make the valve element stop at the intermediate position, but in actuality the valve element is extremely unstable in position. In particular, in a fuel injection system intended to be attached to a heavily vibrating engine, three-position switching type three-way valves using electromagnetic force to position the valve element at the intermediate position are currently not in favor for use. Further, if making a valve element take three positions, the amount of lift of the valve element has to be increased. To increase the amount of lift of the valve element, the electromagnetic coil has to be made considerably larger in size. However, in a fuel injector, making the electromagnetic coil larger is extremely difficult.